The present invention relates to a transmitter of a tire condition monitoring apparatus. More particularly, the present invention pertains to a transmitter for a wireless tire condition monitoring apparatus that permits a driver in a vehicle passenger compartment to check conditions of tires, such as the air pressure.
Wireless tire condition monitoring apparatuses that allow a driver in a vehicle passenger compartment to check the conditions of vehicle tires have been proposed. For example, Japanese Laid-Open Patent Publication No. 9-58230 discloses such an apparatus. The apparatus disclosed in the publication includes transmitters and a receiver. Each transmitter is located in the wheel of one of the tires and the receiver is located in the body frame of the vehicle. Each transmitter wirelessly transmits data, which represents the air pressure and the temperature of the tire detected by a pressure sensor and a temperature sensor, using a carrier wave having a predetermined frequency. Accordingly, each transmitter has an oscillator for generating the carrier wave, such as a quartz resonator or a surface acoustic wave (SAW) resonator. The receiver receives data wirelessly transmitted by the transmitters with an antenna and displays the conditions of the tires, for example, on a display located in the dashboard of the vehicle.
As shown in FIG. 6, each transmitter 300 includes an oscillator circuit 310 that has an oscillator 310. The oscillator 310 generates a carrier wave having a predetermined frequency. The carrier wave oscillated by the oscillator 311 is amplified by a transistor 312 and then sent to a transmission controller.
To perform a stable wireless transmission, the amplitude of the carrier wave must be stable. Methods for maintaining a constant amplitude of a carrier wave include a method for maintaining a constant emitter current. The method utilizes a characteristic of a carrier wave that its amplitude is substantially proportionate to an emitter current. Specifically, in the example of FIG. 6, a resistor 313 for regulating a current is used to maintain a constant emitter current, so that the amplitude of the carrier wave is maintained constant.
However, the temperature in the tire in which the transmitter 300 is located significantly varies depending on the condition where the vehicle is used, or the environmental temperature. Therefore, not only the characteristics of the oscillator 311, but also the characteristics of other electronic components such as the transistor 312, the resistor 313, and a battery 314 are varied according to the temperature. As a result, the amplitude of the carrier wave fluctuates. This makes the carrier wave unstable. To cope with this drawback, high precision electronic components may be used for maintaining a constant amplitude of a carrier wave. However, high precision electronic components would increase the price of the transmitters 300.